The subscribe function must return a function to unsubscribe.

[surejainam]

Bug report

Getting this error even though unsubscribe function is returned

Repro Steps / Code Example

export const subscribeToFirebase = (
  uuid: string,
  onData: (data: UserData | null) => void,
  onError: (error: Error) => void
): (() => void) => {
  // Early exit for authentication
  if (!authService.isAuthenticated()) {
    onError(new Error("Not authenticated"));
    return () => {};
  }

  const db = firebaseService.getDb();
  const docRef = doc(db, MY_COLLECTION, uuid);

  // Set up the snapshot listener - returns unsubscribe function immediately
  const unsubscribe = onSnapshot(
    docRef,
    (snapshot) => {
      const baseData = snapshot.data() as UserData;
      
      if (!baseData) {
        onData(null);
        return;
      }

      // First emit the base data immediately
      onData(baseData);

      // Then process the enhanced data asynchronously
      if (baseData.LN) {
        Promise.all(
          baseData.LN.map(async (atom) => {
            try {
              const rates = await getRates(atom.TYP, baseData.SCR);
              return {
                ...atom,
                MR: rates.market,
                PR: rates.peer,
              };
            } catch (error) {
              console.error('Error fetching rates for atom:', error);
              return atom;
            }
          })
        )
        .then((updatedAtoms) => {
          onData({
            ...baseData,
            LN: updatedAtoms,
          });
        })
        .catch((error) => {
          console.error('Error processing atoms:', error);
          // Data was already emitted, so this error doesn't break the flow
        });
      }
    },
    (error) => {
      console.error('Snapshot subscription error:', error);
      onError(error);
    }
  );

  // Return the unsubscribe function immediately
  return unsubscribe;
};

export const useUserAtoms = () => {
  const { isAuthenticated, error: authError, uuid } = useAuthToken();

  
  const { data, error } = useSWRSubscription<UserData | null>(
    isAuthenticated && uuid ? CACHE_KEY.userAtom(uuid) : null,
    (key: string, { next }: SWRSubscriptionOptions) => {
      console.log('Setting up subscription...');
      
      // Create stable cleanup function
      const noop = () => {};
      
      if (!uuid || !isAuthenticated) {
        console.log('Early return - no uuid or not authenticated');
        return noop;
      }

      let unsubscribe = noop;

      try {
        unsubscribe = subscribeToUserAtom(
          uuid,
          (data) => {
            console.log('Received data:', data);
            if (data) {
              next(null, data);
            } else {
              next(null, null);
            }
          },
          (error) => {
            console.error('Subscription error:', error);
            next(error);
          }
        );

        console.log('Unsubscribe type:', typeof unsubscribe);
        
        // Return cleanup function
        return () => {
          console.log('Cleaning up subscription');
          if (typeof unsubscribe === 'function') {
            unsubscribe();
          }
        };
      } catch (error) {
        console.error('Setup error:', error);
        next(error instanceof Error ? error : new Error('Subscription setup failed'));
        return noop;
      }
    }
  );

  return {
    isAuthenticated,
    atom: data,
    isLoading: !error && !data,
    isError: authError || error || (!uuid && "UUID not provided")
  };
};

Additional Context

SWR version. 2.3.2
NextJS version 15
Add any other context about the problem here.